Opaque quartz glass is used for applications requiring heat shielding properties. The heat shielding properties are related to infrared light-shielding properties, so that the higher the infrared light-shielding properties of opaque quartz glass, the better the heat shielding properties.
Heretofore, as a method for producing opaque quartz glass, a method of adding a blowing agent such as silicon nitride to crystalline silica or amorphous silica, followed by melting, has been known (e.g. Patent Documents 1 to 3). However, opaque quartz glass produced by such a production method has had a problem such that since pores are formed by vaporization of a blowing agent, the average size of pores tends to be large, and with one having practically durable strength, the content density of pores tends to be low, and accordingly the infrared light-shielding properties tend to be low.
On the other hand, a method has also been proposed wherein, without adding a blowing agent, a shaped body of amorphous silica powder is heated at a temperature below its melting temperature, and the heat treatment is interrupted before being completely densified, to be partially sintered (e.g. Patent Document 4). In opaque quartz glass produced by such a production method, it is possible to reduce the average size of the pores, but there is a problem such that if sintering is carried out until pores become closed pores, the content density of pores tends to be low, and the infrared light-shielding properties tend to be degraded, or a problem such that the average size of pores tends to be too small, so that the light shielding properties against infrared light with long wavelengths tend to be low. Further, in such a method, there is a problem such that depending upon the temperature distribution in the electric furnace, a density distribution in the sintered body of opaque quartz glass tends to be formed, whereby it is difficult to obtain homogeneous opaque quartz glass in a large size.
Further, a method of heating and calcining a silica glass porous body under high pressure condition (e.g. Patent Document 5) has also been proposed, but in opaque quartz glass produced by such a production method, the transmittance of light with a wavelength of from 200 to 5,000 nm is from 0.5 to 2.0%, thus indicating a problem that the light-shielding properties against infrared light on the longer wavelength side tend to be low. Further, this method requires a special apparatus for conducting the high-pressure calcination and cannot be said to be a simple method.